Methods and apparatus for electrogasdynamic coating

ABSTRACT

Methods and apparatus for applying coating materials to articles, using electrogasdynamic apparatus providing a flow channel having a dielectric boundary which has a length-/width ratio of greater than 2.5. Gas containing particulate coating material is ionized to create an electrical discharge field for imparting an electrical charge to the particles prior to passage through the dielectrically bounded flow path and thereby creating a high space charge potential. In special applications, a freeradical forming monomer gas or a fusible particle substance is carried in an inert gas into the electrical discharge field and through the flow channel toward the article to be coated. In any case, the charged particles are subjected to an axial charge repelling field (due to space charge effects) in the flow channel to raise the electrical potential of the particles, and thereby the potential gradient between the particles and the workpiece.

United States Patent Gourdine 1 June 27, 1972 [7 2] Inventor: Meredith C. Gourdine, West Orange, NJ.

[73] Assignee: Gourdine Systems, Incorporated, Livingston, NJ.

[22] Filed: June 30, 1969 [21] Appl. No.2 837,562

Related U.S. Application Data [60] Division of Ser. No. 601,270, Nov. 15, 1966, abandoned, which is a continuation-in-part of Ser. No. 512,083, Dec. 7, 1965, abandoned.

[52] U.S. Cl ..317/3, 55/107 [5 1] Int. Cl ..B03c 3/06 [58] Field oiSearch ..317/3,4, 93.4; 310/5, 6; 118/495, 620, 621, 622, 626; 55/107; 239/3, 15

[56] References Cited UNITED STATES PATENTS 3,208,951 9/1965 Berger et a] ..117/93.4 X 3,503,704 3/1970 Marks ..55/107 X Gas 8 Pamela 2,659,841 11/1953 Hampe ..317/3 Primary Examiner-L. T. Hix Assistant Examiner-Harry E. Moose, Jr.

AttorneyBrumbaugh, Graves, Donohue & Raymond [5 7] ABSTRACT Methods and apparatus for applying coating materials to articles, using electrogasdynamic apparatus providing a flow channel having a dielectric boundary which has a length- /width ratio of greater than 2.5. Gas containing particulate coating material is ionized to create an electrical discharge field for imparting an electrical charge to the particles prior to passage through the dielectrically bounded flow path and thereby creating a high space charge potential. In special applications, a free-radical forming monomer gas or a fusible particle substance is carried in an inert gas into the electrical discharge field and through the flow channel toward the article to be coated. In any case, the charged particles are subjected to an axial charge repelling field (due to space charge effects) in the flow channel to raise the electrical potential of the particles, and thereby the potential gradient between the particles and the workpiece.

4 Claims, 4 Drawing Figures Mixture POWER SUPPLY PATENTEnJum m2 I73 Iv l73'a 60$ 8 Particle (1.1%; a" l 17/ nu me E I70 I72 I68 [73d I69 POWER SUPPLY,

I I69 I55 I M POWER Q k SUpPLY Fl G 2 I Airva Powder In/ecti0r// Monomer Gas Injection Air 8 Powder Mixture MEREDITH C. GOURDINE ATTORNEYS METHODS AND APPARATUS FOR ELECTROGASDYNAMIC COATING CROSS-REFERENCE TO RELATED APPLICATIONS This is a division of my copending application Ser. No. 601,270 for ELECTROGASDYNAMIC SYSTEMS AND METHODS, filed Nov. 15, 1966, now abandoned, which in turn is a continuation-in-part of application Ser. No. 512,083, filed Dec. 7, 1965 and now abandoned.

BACKGROUND OF THE INVENTION Electrostatic coating techniques have been used for some time to deposit on a workpiece a homogeneous even coating of charged particles.

The apparatus used to carry out such coating generally consists of a hand-held or mounting spray gun device for atomizing a liquid supply of paint or other coating material, and for subsequently (or simultaneously) charging the coating particles. The particles are then drawn to the coating surface by electrostatic lines of force between the particles and the article. One disadvantage of known methods, however, is the undesirable spreading of the spray pattern, sometimes improved by implementing secondary electrostatic pattem-controlling fields.

Another disadvantage of such known apparatus is their failure to generate sufficient space charge (particle) potential to establish effectively strong field gradients between the particles and the surface of the object to be coated. To create strong gradients, it has been the practice to use electrostatic atomization, or to impose an external field (using extremely high voltages) between the spray source and the coating object. Where high external electric fields are used, the possibility of arcing is enhanced, necessitating sometimes elaborate safety precautions.

SUMMARY OF THE INVENTION The present invention offers improved techniques and apparatus for applying a variety of particulate coating substances to surfaces by charging coating particles and passing them through a substantially, non-conducting flow path boundary to increase the space charge potential due to the electrical charge on the particles. Preferably, the flow channel has a minimum aspect ratio of length/width of 2.5, and the particles to be charged may be liquifiedby heat prior to charging, or can be formed by subjecting a monomer gas to an electrical discharge.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of these and other aspects of the invention, as well as the objects and advantages thereof, reference may be made to the following detailed description and to the drawings, in which:

FIGS. 1-3 are schematic illustrations in cross section, showing various applications of electrogasdynamic ionizing apparatus in accordance with the invention to the coating of object surfaces; and

FIG. 4 is a cross-sectional view of a two-channel electrogasdynamic ionizer in accordance with the invention, in which the outlets of the channels are arranged to intermix the fluids exiting therefrom.

The principles of electrogasdynamic precipitation may be utilized in the coating of object surfaces, and because of the versatility and simplicity of electrogasdynamic apparatus, EGD coating systems may be operated on gas streams seeded with both organic and inorganic materials. Moreover, both insulative and metallic surfaces can be coated. EGD precipitation techniques are applicable to coating techniques because, in effect, the object to be coated serves as a collector electrode. Moreover, as in EGD precipitators, it is desirable to charge particles and create in a flow channel a high space charge potential without, at the same time, causing dielectric breakdown of the gas or coating aerosol.

In the above-noted application Ser. No. 601,270,EGD apparatus are disclosed, all of which have an aspect ratio of length (L) to width (W) exceeding 2.5, where the width W is taken as the maximum cross-sectional dimension of the channel (normal to the direction of flow) at the maximum space charge concentration. Altemately, for flow channels of cylindrical cross-section, the aspect ratio is expressed as L/R, where R is the cylinder radius; and the corresponding minimum aspect ratio is 5.

It has been found that similar limitations apply to EGD coating apparatus, where the flow channel should have an aspect ratio of UK greater than 5, or L/W greater than 2.5 in order to achieve optimum space charge potentials without incurring dielectric breakdown. These aspect ratios take into account not only the maximum safe operating potentials for most coating operations, but also the maximum desirable transverse electric field within the channel due to space charge alone, while allowing for sufficient length to carry the charged particles away from the ionizing electrodes toward the workpiece.

The schematic illustration of FIG. 1 depicts the use of an electrogasdynamic ionizer 164 in a system for coating solid objects positioned within a chamber 165. The ionizing apparatus is in the form of a gun and consists of a single flow channel defined between parallel dielectric plates 166, corona and attractor electrodes 167 and 168, respectively, and a power supply 169 for applying an ionizing potential between the electrodes 167 and 168. The electrodes are of the type shown and described in detail in application Ser. No. 601 ,270.

The gun 164 is supported by a suitable structure, such as the conduit 170 having an inlet 171 receiving a carrier gas in which a coating substance in particle form is entrained. The gas and particle mixture enters the electrogasdynamic fiow channel inlet 172 where it becomes charged in the electrical discharge field between the corona and attractor electrodes 167, 168. Ions and charged particles move downstream and are exhausted into the chamber 165, creating a space charge field. However, because the channel is relatively long, the charged particles exit a sufficient distance downstream of the electrode to inhibit the tendency of the particles to return to the gun structure at a lower electric potential.

For purpose of explanation, the chamber 165 is shown to contain three sold objects of arbitrary shape, the first object 173 being metallic and connected by a conductor 173a to a metal wall 165a of the chamber, which is referenced to ground. Another object l73b is of a dielectric material and is referenced to ground by the conductor 173e, which terminates internally of the mass 173b. Finally, a third object 173d, constructed of either metal or an insulative material, has no connection to a reference potential. As the charged particles are emitted from the gun 164 into the chamber 165, the objects 173 and l73b will both receive an even coating over their total surfaces, irrespective of the direction in which the surfaces of these objects face, since the space charge field in the chamber 165 is effective to induce the charged particles to seek a potential lower than the space charge potential. The object 173d, on the other hand, will receive a coating only on those surfaces exposed directly to the flow of particles from the outlet of the flow channel in the gun 164.

In FIG. 2 the electrogasdynamic gun 164' is shown supported by suitably resilient guides 175 at the interior of a conduit or pipe 176 of which the inner surface 176a is to be coated. An air and dry powder mixture is fed into a pump or compressor 178 and subsequently into a heating unit 179 where the individual particles are liquified by the application of heat. From the heater 179, the air and liquified particles FIG. 3 shows an extension of the foregoing principles to the coating of a flat object 180. There, the electrogasdynamic apparatus includes a pair of dielectric plates 181, 182, with the longer of the plates 182 serving to direct the flow over the surface 180a. Further, as charged particles are blown into the space between surface 180a and the plate 182, the space charge field drives some of the charged particles to the dielectric plate 182. This plate soon reaches a condition of charge saturation and produces an electrical field gradient normal to the plate 182 in a direction tending to aid the space charge field in depositing the particles on the surface 180a.

At the inlet to the electrogasdynamic gun, an aerosol, such as an air and dry powder mixture of a desired material, is intermixed with a free radical-producing gas, e.g., a monomer gas. In the ionizer 183, the powder particles are charged and the monomer gas is broken down into free radicals, both charged and uncharged. The charged aerosols, along with the uncharged free radicals, are carried downstream into the collector" which, in the FIG. 3 device, is formed between the dielectric plate 182 and the surface 180a of the object to be coated. By precipitating action, (as well as by diffusion), the uncharged free radicals and charged aerosol particles are deposited on the surface 180a, where the free radicals polymerize to form a thin film coating and assist in the fusion of the powdered particles As an example, the monomer gas may be ethylene and the powder polyethylene. When the ethylene free radicals and polyethylene powder are deposited on the surface 180a, they form a thick film without the application of heat.

As a further example, a monomer gas alone may be used for surface coating. In such case, an inert gas, such as argon or neon, may be used as a carrier for the free radicals formed in the corona discharge of the EGD gun. Molecular ions and the charged free radicals formed in the corona discharge are not carried downstream by the flow to any appreciable distance, since they possess relatively high mobilities and are quickly attracted by the attractor electrode in the ionizer 183. Therefore, primarily only the uncharged free radicals exit into the collecting region. This phenomena is of advantage, since after an initial coating is built up on the object surface, charged particles thereafter deposited can burn the coating by discharging through the coating to the coated surface. It will be appreciated, that any of a number of monomer gases, such as styrene and propylene, may be employed.

The device of FIG. 4 operates identically to those shown in FIG. 1 to charge the particles dispersed in two separate thin flow channels 185a, 185b formed by the parallel dielectric plate structure 186, 187, 188. Each plate has associated therewith a plate attractor electrode 189, all of which can be electrically connected to be at the same potential, viz, the potential on the conductor 190. Ionization excitation sources 191a, l9lb of opposite polarity are attached between the common conductor 190 and respective corona electrode arrays 192a, 192b in the flow paths to yield ionization fields of corresponding opposite electrical charges. As depicted, the gas in the channel 185a becomes positively ionized and the particles carried thereby positively charged, while the gas and particles flowing through the channel l85b are associated with negative charges.

At the exit end of the device, the flow channels 1854, 185b are directed toward each other in such a way that the flow through the one channel intermixes with the flow through the other channel. Thus, at the flow channel exits, the positively and negatively charged particles are in close proximity whereby they are mutually attractive. If, for example, particles carried in the stream in the channel 185a are liquid and those particles in the channel 185b are solid, the (negative) solid particles become coated with the (positive) liquid particles, the charge on at least one of the attractive particles becoming neutralized upon physical combination. It is apparent, of course, that the apparatus of FIG. 4 can be used in any manner according to the invention to bring about charging and combining of gas entrained particles in whatever physical state, whether liquid or solid. Moreover, the device is further advantageous in efiecting ionization of the gas flowing in each of the separate flow channels whereby ions of the ionized gas become mutually attractive until chemical or electrical combination occurs. Accordingly, the term particle" is used in its broadest sense.

The foregoing principles may be applied to the formation of a thin sheet of material by stripping the coating from the coated object in a well-known manner. For example, in FIG. 3 the flat object might represent one run of an endless belt so that the surface a thereof moves past the particle discharge. Subsequently, the coating may be peeled from the surface 1804 by a knife-edge (not illustrated) to form a continuous sheet of the coating material.

I claim: 1. In an electrogasdynamic system for applying a coating to an article, the combination of:

means for forming a suspension of coating particles in a gaseous stream; electrode means for subjecting the coating particles to an electrical discharge to impart an electrical charge to the particles; and flow channel means defining for the charged particles a flow path having a substantially non-conducting boundary which has an axial length between the electrode means and an outlet that is greater than the maximum width thereof, at an axial location of greatest charge concentration, by a factor of at least 2.5. 2. Apparatus according to claim 1, further comprising: source means of at least one monomer gas; and means for introducing the monomer gas into the gaseous stream to be subjected to the electrical discharge. 3. Apparatus according to claim I, further comprising: means for applying heat to the suspended particles prior to deposition thereof on the article and sufi'rcient to condition the particles for fusion upon contact with the article. 4. Apparatus according to claim 1, further comprising: gas supply means for moving the gaseous stream and the charged particles through the flow channel means, and; the flow channel means opening into a zone less restricted in cross section than the flow channel means to establish a space charge field by the emission of charged particles therein, and; the means for moving the gaseous stream and the charged particles being operative to force the particles against an axial charge repelling field resultant from the space charge field and into the space charge field to significantly increase the potential on the particles.

t 0' II I l 

1. In an electrogasdynamic system for applying a coating to an article, the combination of: means for forming a suspension of coating particles in a gaseous stream; electrode means for subjecting the coating particles to an electrical discharge to impart an electrical charge to the particles; and flow channel means defining for the charged particles a flow path having a substantially non-conducting boundary which has an axial length between the electrode means and an outlet that is greater than the maximum width thereof, at an axial location of greatest charge concentration, by a factor of at least 2.5.
 2. Apparatus according to claim 1, further comprising: source means of at least one monomer gas; and means for introducing the monomer gas into the gaseous stream to be subjected to the electrical discharge.
 3. Apparatus according to claim 1, further comprising: means for applying heat to the suspended particles prior to deposition thereof on the article and suFficient to condition the particles for fusion upon contact with the article.
 4. Apparatus according to claim 1, further comprising: gas supply means for moving the gaseous stream and the charged particles through the flow channel means, and; the flow channel means opening into a zone less restricted in cross section than the flow channel means to establish a space charge field by the emission of charged particles therein, and; the means for moving the gaseous stream and the charged particles being operative to force the particles against an axial charge repelling field resultant from the space charge field and into the space charge field to significantly increase the potential on the particles. 